81 Wineglass Flow

This embraces only the small dacite flow, about three-quarters of a mile long, that starts near the rim at the Wineglass and extends northeastward. In the hand specimen the rock (114) closely resembles the brown and black vitrophyric dacite from the Cleetwood Cove region, just above described. In this case, however, the black glassy portions appear more as elongated streaks accentuating the fluidal structure.

Under the microscope the darker portions that appear black in the hand specimen are seen to be composed of nearly pure glass, but not of a uniform color. The rock consists of more or less parallel strands of absolutely colorless glass and of glass rendered more or less opaque by means of blackish to brownish minutest dust-like particles of uncertain character. The usual microlites of augite appear to be missing. This interweaving of colorless and dark-colored strands causes the glass to assume a decided stringy appearance. In case two phenocrysts approach each other the strings or strands are apt to assume almost or entirely parallel directions, but elsewhere they show a constantly varying arrangement, frequently becoming wavy or strongly crinkled, like the crinkling of many chlorite and mica schists. Furthermore, this stringy structure does not run uninterruptedly through the thin section, but is broken up into more or less separate areas that have lenticular or twisted forms. In general the arrangement of these glassy strands corresponds with the customary fluidal structure of glassy rhyolites and dacites, in that the strands lap or flow around the phenocrysts, after the manner of strings of microlitic inclosures.

The red portion of this dacite does not appear to differ in thin section from the above-described black areas, except that it is stained with dirty looking, brownish, powdery, ferritic matter. It constitutes the greater part of the rock and appears to inclose irregular, frayed out areas of the darker colored glass. This partial alternation of different colored areas lends a brecciated appearance to the whole.

The characteristic structure of this dacite does not differ essentially from similar structures as described by observers elsewhere, but it reminds the writer forcibly of the red obsidian which Iddings has described, and of which he has given a colored reproduction.a The phenocrysts of this dacite are not noticeably different from those in the dacites already described. They consist of plagioclase and hypersthene and of isolated, minute, reddish-brown hornblende. The rock also contains small fragments of a holocrystalline, porphyritic, hypersthene-bearing basalt, and in one case at least of a hyalopilitic andesite. These fragments are with a few exceptions hardly noticeable in the hand specimen, as they are small, but they are very numerous, as each of the two thin sections prepared from this rock contains eight or nine such inclosures that measure from 2 or 3 millimeters down to about 0.02 millimeter in diameter.

aSeventh Ann. Rept. U. S. Geol. Survey, 1886, p. 1.

The chemical analysis of this dacite will be found on page 140. It will be noted that the amount of silica is 2 or 3 per cent less than in the two other dacites analyzed. Unfortunately the presence of the above-mentioned inclosures of basalt vitiate to some extent the analysis. The lower percentage in silica may easily be explained by the presence of these inclosures, which may have been present in the portion sent for analysis to an even greater extent than is indicated by the thin sections.

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